The present invention concerns contacting a hydrocarbon feed with a new reforming catalyst which has a superior selectivity and activity for dehydrocyclization, isomerization, and dehydroisomerization.
Catalytic reforming is well known in the petroleum industry. It involves treating naphtha fractions to improve the octane rating by producing aromatics and isomerizing normal and singly branched paraffins. The more important hydrocarbon reactions occurring during reforming operation include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of acyclic hydrocarbons to aromatics, dealkylation of alkylbenzenes, isomerization of paraffins, and hydrocracking reactions which produce light gaseous hydrocarbons, e.g., methane, ethane, propane and butanes. Hydrocracking reactions should be minimized during reforming as they decrease both the yield of products in the gasoline boiling range and the hydrogen.
Because of the demand for high octane gasoline for use in motor fuels, extensive research is being devoted to developing improved reforming catalysts and catalytic reforming processes. Catalysts for reforming processes must be able to produce high yields of liquid products in the gasoline boiling range (containing large concentrations of high octane number aromatic hydrocarbons) and low yields of light gaseous hydrocarbons. The catalysts should possess good activity in order that the temperature required to produce a certain quality product need not be too high. The catalysts should also either possess good stability, in order that the activity and selectivity characteristics can be retained during prolonged periods of operation, or be sufficiently regenerable to allow frequent regeneration without loss of performance.
Catalysts comprising platinum, for example, platinum and rhenium supported on alumina, are widely used for the reforming of naphthas.
The use of carriers other than alumina has been studied and it was proposed to use certain molecular sieves such as X and Y zeolites, which have pores large enough for hydrocarbons in the gasoline boiling range to pass through. However, reforming catalysts based upon these molecular sieves have not been commercially successful.
In conventional reforming, the hydrocarbons to be converted are passed over the catalyst, in the presence of hydrogen, at temperatures of about 450.degree. C. to 550.degree. C. and pressures of about 50 to 500 psig. Part of the hydrocarbons are converted into aromatic hydrocarbons, and the reaction is accompanied by isomerization and cracking reactions which also convert the paraffins into isoparaffins and lighter hydrocarbons.
The catalysts hitherto used have given fairly satisfactory results with heavy paraffins, but less satisfactory results with C.sub.6 -C.sub.8 paraffins, particularly C.sub.6 paraffins. Catalysts based on a type L zeolite are more selective with regard to the dehydrocyclization reaction and produce excellent results with C.sub.6 -C.sub.8 paraffins.
The selectivities of these catalysts for dehydrocyclization are so great that little isomerization and direct dehydroisomerization occurs. While it is highly desirable to reduce the amount of hydrocracking occurring in reforming, some isomerization is desirable to convert unreacted straight-chain and singly branched paraffins to isomers having higher octane numbers. While cyclopentanes can be converted to aromatics by ring opening followed by dehydrocyclization, a more favored route is to go directly to aromatics by dehydroisomerization.